This invention is related to laser transmitters and more particularly to a second order predistortor for such return laser transmitters.
Optical transmitters are typically utilized in communication systems for receiving electrical signals and transforming them into optical signals. The electrical signals can be radio frequency (RF) signals being transmitted within the communication system. An example of such a communication system is a CATV network.
A CATV network typically consists of a downstream path extending from a service provider location known as a head end to various nodes through taps down to settop terminals located at subscriber locations. The upstream path extends from the settop terminals back to the head end. These communication systems can be designed to be partially optical and partially electrical. For example, communications between the headend and the nodes in both the upstream and downstream directions can be accomplished utilizing optical signals while communications in both directions between the nodes and settop boxes can be electrical. Such a system requires optical receivers and transmitters to be located both at nodes and at the headends.
Return optical transmitters are necessary at each node transmitting optical signals to the headend. Likewise, optical receivers are necessary at the headend to receive optical communications from the nodes traveling along the upstream path.
Since CATV systems are becoming increasingly bidirectional, bandwidth requirements along the upstream path are also increasing. The increased bandwidth requirements are attributable to the need for upstream communications associated with Internet access, fax capabilities, pay per view, and other upstream information transfer.
With the increasing bandwidth requirements comes a need to minimize second order distortion levels in the optical transmitters. Since the optical transmitters located at nodes are typically disposed along the network in outdoor environments, they are subject to large temperature variations. For example, ambient temperature may range from xe2x88x9240C. to +80C. These temperature variations effect laser transmitter performance and cause variation of second order distortion levels which may be between xe2x88x9240 dBc and xe2x88x9260 dBc being introduced by the lasers within the optical transmitters at various temperatures. A predistortion solution for a return transmitter, therefore, must have a controllable predistortion level mechanism that can be adjusted according to temperature changes.
The invention addresses the above mentioned problem by providing a method and circuit for reducing distortion levels in an optical return transmitter. An RF input to the optical transmitter is first entered at an input stage. An emitter follower stage is connected to the input stage to introduce a desired level of second order predistortion in the RF signal. The desired predistortion level is controlled by controlling the level of a D.C. current applied to the emitter follower stage.